The present invention is directed to and proceeds from an apparatus for the EMC testing of electrical devices.
In prior art apparatus for EMC (electromagnetic compatibility) or, respectively, EMI (electromagnetic interference) measurement the electromagnetic field in the chamber was generated exclusively by an asymmetrical excitation system. The metallic chamber forms the outer conductor and a metal plate, insulated in the chamber, forms the inner conductor of a coaxial line system that is asymmetrically supplied from a radio frequency source (see Patent references DE 39 31 449, DE 31 30 487, DE 39 25 247, DE 37 31 165, DE 43 00 778, EP 0 517 992; also see Wunderlich, Werner, Storfestigkeitsmessungen an Einrichtungen zur drahtlosen Nachrichtenubertragung mit relativ grossen Abmessungen, Technische Mitteilungen des RFZ, Vol.28, No.1, 1984, pp.220 ff.; also see Crawford, M. L., Workman, J. L., Thomas, C. L., "Generation of EM susceptibility Test Fields Using a Large Absorber-Loaded TEM Cell", IEEE Transactions on Instrumentation and Measurement, September 1977, No.3, pages 225 ff.). In order to achieve a uniform field strength distribution in the chamber as required by the EMC norm (the norm requires that the change in field strength in the testing chamber is less than 6 Db), the inner conductor plate on which the unit under test is placed must be arranged in the middle of the chamber insofar as possible. A result, however, only half the space of the chamber is available for the acceptance of the unit under test. Moreover, the EMC testing apparatus that meet the norm are very bulky since a pyramidal chamber design is required for the transition from the coaxial feeder cable to the system of inner conductor and outer conductor that generates the field.
It has been attempted to arrange the inner conductor as close as possible to the lower bottom wall of a quadratic chamber (G-STRIP cell Comtest of the Thermo Voltek company, Great Britain). With this approach the usable space for the acceptance of the unit under test is enlarged, but this known cell does not meet the strict norm for the uniform field distribution in the chamber.